Since a nitride (GaN) semiconductor device, specifically, a nitride field effect transistor, is a direct transition type semiconductor device and has a high electron moving speed (2×107 cm/s) and a high breakdown field (3×106 V/cm), the nitride field effect transistor is attractive as a new high-frequency electronic device. Further, since the nitride field effect transistor can be implemented in a heterojunction structure such as AlGaN/GaN and can be selectively doped, the nitride field effect transistor can be designed in an optimal structure for high speeds. The nitride field effect transistor is coming up as a new semiconductor device having high voltage and high frequency properties by significantly improving a trade-off relationship between a cut-off frequency ft and a breakdown voltage VBV which were problematic in a field effect transistor in the related art.
When the nitride field effect transistor (FET) having high power density in addition to the high voltage and high frequency properties is used, peripheral circuits, such as a power distributing and combining circuit and a DC voltage converting circuit, can be removed or simplified, and as a result, a high power amplifier module having can be configured.
Meanwhile, for the high-speed nitride field effect transistor, a T-gate, a Y-gate, or a mushroom-gate electrode having a large cross-sectional area is essentially used to improve noise property by reducing high modulation operation and gate resistance. The T-, Y-, and mushroom-gate electrodes are generally formed through an electronic beam lithography method or a photolithography method. Since resolution deteriorates while minutely forming a critical dimension of the gate electrode using the photolithography method, the electronic beam lithography method is more used, in which a photoresist film of a double layer or a triple layer is generally used.
The nitride field effect transistor will be described below in detail with reference to FIG. 1.
FIG. 1 shows a nitride field effect transistor having a field plate electrode in the related art. The nitride field effect transistor in the related art includes a source electrode 120 and a drain electrode 130 that are spaced apart from each other on a AlGaN/GaN heterojunction epiwafer 110; a passivation layer 140 formed on epiwafer 110 between source electrode 120 and drain electrode 130 and including a contact hole; a gate electrode 150 connected with epiwafer 110 through the contact hole formed in passivation layer 140; a first field plate electrode 160 formed on passivation layer 140 to connect gate electrode 150; and a second field plate electrode 170 formed on passivation layer 140 to be spaced apart from first field plate electrode 160.
However, in the field plate electrode structure in the related art, since first field plate electrode 160 is also formed simultaneously while forming gate electrode 150, it is difficult to manufacture a gate electrode having a minute pattern width and it is problematic in connecting the gate electrode and the field plate electrode to each other, and as a result, the gate electrode may collapse, thereby deteriorating transistor property. Further, the spacing distance between first field plate electrode 160 and second field plate electrode 170 needs to be adjusted and since first field plate electrode 160 and second field plate electrode 170 are formed by an image reversal process, the adjusted spacing distance may vary, such that the performance of the transistor may be problematic.